Antilock brake control



Dec. 15, 1970 J. L, HARNED ETAL ANTILOCK BRAKE coNTR Original Fild Aug.12, 1968 3 Sheets-$heet 1 30 To ATMOSPHERE F0 5% w E VEHICLE ELECTRONICWHEEL TAcHoMErER CONTROL )5 DRIVE ClRCUIT V VACUUM a ,souRcE w 68 u/ E4276 REVERSE m DIFFERENTIAL DRWE $9 MI, W a? 55%! u a? Z 1 Mil; 0- '3 ZINVENTORS. I

ATTORNEY Dec. 15,1970 ,1, WM Ema ANTILOCK BRAKE CONTROL Original FiledAug. 12, 1968 3 sheets shaat.

2 I I i I g? 1/200 i 114 -Haa v l l I d, X '2 562 A04 1! Z lDISCRIMIINATOR C, Km i I Dec.

3 Sheets-Sheet 3 ATTORNEY s r R 0 r. M 0 e a m w W b wflfi nm @M f Y B Mw United States Patent 3,547,501 ANTILOCK BRAKE CONTROL John L. Harned,Grosse Pointe Woods, and Edwin E. Stewart, Warren, Mich., assignors toGeneral Motors Corporation, Detroit, Mich., a corporation of DelawareOriginal application Aug. 21, 1968, Ser. No. 754,252. Divided and thisapplication Dec. 12, 1969, Ser.

Int. Cl. B60t 8/08 US. Cl. 30321 6 Claims ABSTRACT OF THE DISCLOSURE Anelectronic control is responsive to wheel acceleration to producesignals for effecting alternating brake release, hold, and applicationto avoid wheel lock. A wheel velocity error circuit compares a wheelacceleration signal to a reference signal and integrates the differenceto control brake release. A similar circuit responsive to the wheelacceleration signal and another reference signal controls brake hold andapplication. A third circuit varies the reference signals to optimizethe control function.

This is a division of application Ser. No. 754,252 filed Aug. 21, 1968.

This invention relates to a control for a vehicle brake system of thetype which maintains near optimum braking torque to avoid wheel lock-up.

It has previously been proposed in antilock brake controls to comparethe velocity of a braked wheel to a preprogrammed reference velocity,such as the velocity of an inertia wheel or the like and to effectbraking as a function of some fixed relationship between the actual andthe reference velocities and accordingly, such controls lack thenecessary flexibility to attain optimum effectiveness in all ranges ofoperation. Generally, such controls have an inherent time lag betweenthe attainment of the critacal velocity relationship and the generationof an output signal for effecting brake release. Such time lags,although minute, can have deleterious effects on system performance.Such prior controls have also generally provided incipient lock-updetection means sensitive to electrical noise or other spurious signalsand subject to loss of control in the event that wheel lock-up doesoccur.

Other such controls have been proposed which do have a rapid responsebut which require an expensive proportional valve to control brakepressure in response to a control signal.

It is a general object of this invention to provide in an antilock brakecontrol system an electronic control having reference parameters whichmay be varied to optimize system performance.

It is another object of the invention to provide in an antilock brakecontrol system a circuit for providing a variable deceleration referencesignal for insuring the wheel is in an incipient lock-up conditionbefore the control circuit becomes effective to modulate the brakepressure.

It is a further object of the invention to provide in an antilock brakesystem an electronic circuit to control brake pressure in arelease-hold-apply mode including an interlock circuit for disabling abrake release signal when a brake hold signal is applied.

The invention is carried out by providing a circuit for ice programminga variable reference signal which is a function of a desired wheeldeceleration, a circuit for producing a signal which is a function ofactual wheel deceleration and a further circuit for comparing thesignals and for producing an output signal which represents the amountthat the desired wheel velocity exceeds the actual wheel velocity.

The invention is further carried out by providing in an antilock brakesystem as electronic incipient wheel lock-up sensing circuit having asan input a signal proportional to wheel acceleration and having meansfor providing a variable reference signal which is a function of optimumdeceleration and a circuit for summing and integrating the signals toproduce an output which is a function of the difference between thewheel velocity and a velocity which is a function of the referencesignal, the output being adapted to effect a partial release of brakepressure. The invention further contemplates including in the system anelectronic circuit responsive to the acceleration signal and to avariable reference signal which is a function of optimum accelerationafter brake release for integrating the signals to produce an outputwhich is a function of the difference between the wheel velocity and avelocity which is a function of the variable reference signal, theoutput being adapted to cause the brake pressure to hold at a reducedconstant value until the wheel velocity recovers from an incipientlock-up condition and then to effect reapplication of brake pressure.

The above and other advantages. will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts andwherein:

FIG. 1 is a diagrammatic representation of an antilock brake controlsystem according to the invention;

FIG. 2 is a diagrammatic representation of a mechanical analog of theinertia wheel velocity reference princip FIG. 3 is a functional diagramof the mechanical analog of FIG. 2;

FIG. 4 is a logic and block diagram of the electronic control circuitaccording to the invention;

FIG. 5 is a graph depicting various operating parameters of the controlsystem of FIG. 4; and,

FIG. 6 is a schematic diagram of the circuit of FIG. 4 according to theinvention.

The system in which this invention is utilized follows the wellestablished principle of sensing incipient wheel lock-up when brakepressure is applied to vehicle brakes, then relieveing the brakepressure until the wheel accelerates enough to be out of danger oflocking and then reapplying the brake pressure. This cycle is repeatedas necessary to achieve the desired braking action. This system isparticularly well adapted to practice the control principle of theextremal type in which brake pressure sufficient to cause asubstantially increased wheel slip is released to permit wheelacceleration and consequent decreasing wheel slip. The brake pressure isthen held at a value to permit wheel acceleration and therefore adecrease in wheel slip while maintaining a brake torque on the wheeluntil the wheel acceleration ceases. The brake apply pressure then againin increased to cause wheel deceleration. This extremal type of controlis more fully set forth in the patent application U.S. Ser. No. 658,420filed Oct. 4, 1967 by D. M. Florry.

Referring to FIG. 1 for a further description of the general system, avehicle wheel drive 10 is provided as an information source for thesystem. The vehicle wheel drive may be a wheel per se, a propeller shaftdriving a plurality of wheels or any other vehicle member having avelocity or rotation proportional to wheel velocity. A tachometer 12 isdriven by the wheel drive 10 and provides an input signal to anelectronic control circuit 14. The electronic control circuit 14 in turnfurnishes two signal outputs on conductors 16 and 18. A brake pressuremodulating system comprises a combination pneumatic and hydraulicsystem. The pneumatic portion of the system comprises a tube 20 ventedto atmosphere at one end and connected to a vacuum source 22 which maybe the vehicle engine manifold at the other end. The tube 20 contains inseries a restricting orifice 24 near the vent to atmosphere, a pressureaccumulator 26, a normally closed electrically operated release valve 28controlled by the output signal on conductor 16 and a second restrictingorifice 30. A branch tube 32 is connected to the tube 20 at a pointbetween the valve 28 and the orifice 30 and leads to a pressuremodulator 34. The branch tube 32 contains a normally open electricallyoperated hold valve 36 controlled by the output signal on conductor 18.Still another pnenumatic tube 38 connects the modulator 34 with thevacuum source. The hydraulic portion of the system includes a manuallyoperated conventional master cylinder 40 for supplying brake pressure toconventional vehicle brakes 42 through a first supply line 44, themodulator 34 and a second supply line 46. The structure and operation ofthe modulator 34 is fully described in the above-mentioned Florryapplication and no further description herein is necessary except topoint out its function. Normally vacuum pressure from the source 22 isapplied through the tubes 38 and 32 to both sides of the modulater 34and in this condition, the modulator 34 transfers the brake pressurefrom the master cylinder 40 to the brakes 42 without any modulation.However, when air pressure is supplied from atmosphere to the modulator34 through the tubes 20 and 32, the modulator will first isolate thebrakes 42 from the master cylinder 40 and will then relieve pressurefrom the brakes 42 to a degree determined by the amount of air pressuresupplied through tube 32. When the hold valve 36 is closed the modulator34 will hold the brake pressure constant. If the valve 28 is then closedand the valve 36 is opened, the brake pressure will tend to assume themaster cylinder presure. In operation of the system in general, a wheelvelocity signal is supplied to the electronic control circuit 14 by thetachometer 12. During normal braking of the vehicle, the electroniccontrol circuit is ineffective to cause any modulation of pressureapplied to the brakes 42. However, a during a panic braking condition,the applied braking pressure is so great that the wheel begins toapproach a lock-up condition and the electronic control circuit 14produces an output on conductor 16 causing the valve 28 to open. Then,air from the accumulator 26 rapidly enters the modulator 34 to quicklyrelieve some of the pressure on the brakes 42 and then additional airbleeding through the orifice 24 effects further relief of brakepressure. The resulting decrease of brake pressure allows the wheel toaccelerate and when such acceleration begins, the electronic controlcircuit 14 produces an output on the conductor 18 to close the valve 36,thereby preventing further air pressure change in the modulator 34 andcausing the modulator 34 to maintain a constant brake presure. Finally,as the wheel velocity recovers, the electronic control circuit 14removes the signals from conductors 16 and 18 to close the release valve28 and open the hold valve 36 so that the air pressure within themodulator 34 will bleed off through the restricting orifice 30 to thevacuum source 32 to allow an increase of brake pressure and a wheeldeceleration to initiate the cycle again if necessary.

Inertia wheel velocity reference principle The electronic controlcircuit 14 utilizes a control principle called herein the inertia wheelvelocity reference (IWVR) principle which provides precise control andrapid response. The IWVR principle is illustrated in FIG. 2 by amechanical analog wherein a shaft 60 rotates with a wheel 62 at avelocity to of the wheel. An inertia wheel or flywheel 64 is mounted onthe shaft 60 by an overrunning clutch 66 so arranged that the inertiawheel 64 will be driven by the shaft 60 when the wheel velocity to isincreasing. The inertia wheel 64 is further driven by the shaft througha slip clutch 68 having a driving member 69 secured to the shaft 60. Theslip clutch 68 is of the type which transmits a maximum torque T and ifthat toryque T is exceeded slipping will occur between the drivingmember 69 and the inertia wheel 64. With this arrangement then, thevelocity of the inertia wheel 64 will always follow the velocity of thewheel 62 when the latter is accelerating and will also follow thevelocity of wheel 62 when the latter is decelerating, providing that thedeceleration does not create a torque in the slip clutch in excess ofthe value T. When deceleration of the wheel 62 does exceed that value,the inertia wheel 64 will overrun the shaft 60 but it will decelerate ata constant rate due to the constant torque T or drag applied thereto bythe slip clutch 68. The deceleration rate of the inertia wheel 64represents the optimum wheel deceleration within the design limitationof the system. Similarly, the velocity of the decelerating wheelrepresents the optimum wheel velocity profile within design limitations.The shaft 60 carries a gear 70 and the inertia wheel 64 carries a gear72. These gears 70 and 72 are connected to a differential 74 throughsuitable gear trains comprising gears 76, 78 and a reverse drive 80. Thedifferential 74 has an output 82 which will rotate at a velocity Awwhich is the difference between the wheel velocity u) and the velocityof the inertia wheel. Thus, when the wheel 62 and the inertia wheel 64rotate in synchronism, the differential output Aw, will be zero. Whenthe velocity of the inertia wheel 64 exceeds that of the wheel 62 Awwill equal the difference in velocities. It will be seen then that thetorque T of the slip clutch 68 can be so selected that during normalbraking Aw will equal zero, but during panic braking when the wheel 62experiences abnormal deceleration, the slip clutch 68 will slip and Awwill assume a finite value Aw which then can be used as a controlparameter. It should be noted that the value Aw is instantaneouslyresponsive to abnormal deceleration of wheel 62 and reflects thedeviations of the wheel velocity (.0 from the programmed referencevelocity of the inertia wheel 64. For example, should wheel 62 come toan abrupt stop, Aw will instantly assume a value equal to the velocityof the inertia wheel 64 and, so long as the wheel 62 remains stopped, Awwill gradually decrease as the velocity of the inertia wheel 64decreases due to the torque T.

A functional diagram of the mechanical analog is depicted in FIG. 3.This functional model is mathematically equivalent to the mechanicalanalog of FIG. 2. The input is in the form of the acceleration 5., ofthe wheel 62 and combined at a summer 84 with another deceleration inputm which corresponds to the value of torque T of the slip clutch 68divided by the inertia of wheel 64 and represents the optimum wheeldeceleration during braking. The output of summer 84 is fed through asecond summer 86 to an integrator 88. The output of the integrator then1s Aw A feedback of Aw to the summer 86 through a limit stop 90 assuresthat the value of Aw cannot go positive. The limit stop 90 thereforerepresents the overrunning clutch. It will thus be seen that startingfrom a condition of w=0, when 5, becomes positive, the integrator outputAw will remain zero due to the action of the limit stop 90. When, (I; isnegative but does not exceed the value of a Aw still remains zero. If,however, 1, becomes more than on, the difference between I, and 11 willbe integrated to produce a finite value Aw and Aw will be generatedwithout delay.

Electronic control system For clarity, the electronic control system isfirst shown in FIG. 4 in the form of a block diagram and is again shownschematically in FIG. 6. The electronic control circuit 14 comprises aninput circuit 100, a wheel velocity error circuit 102, and a controllogic circuit 104 and a pressure hold circuit 106. A wheel speed sensoror tachometer 12 such as a well known toothed wheel variable reluctanceelectromagnetic transducer produces an alternating output having afrequency proportional to the wheel speed. The alternating signal ischanged to a DC signal by a discriminator 108 which is afrequencyto-voltage converter and is smoothed by filter 110 to produce aDC signal to having an amplitude proportional to frequency and hence towheel velocity. The signal to is fed to a diiferentiator 112 whichprovides an output of wheel acceleration 1,. The output I, is fed to thewheel velocity error circuit 102 and particularly to a summer 114 whereit is combined with a deceleration threshold signal a The decelerationthreshold signal a like the torque T of the mechanical analog,represents (within the design limitations of the system) the optimumwheel deceleration during braking, and of course is the time derivativeof the optimum wheel velocity profile. The composite output of thesummer 114 is then supplied via a summer 116 to an integrator 118 whichproduces the output Aw A feedback from the integrator 118 output to thesummer 1 16 has a limit stop 120 for the purpose described previously inthe discussion of FIG. 3. As will be seen, in practice a separate limitstop circuit 120 is unnecessary where the integrator 118 includes anoperational amplifier which inherently has both positive and negativesaturation values to thereby provide a limit stop. The output ofintegrator 118, Aw is fed to a hysteresis circuit 122 which, as showndiagrammatically, produces an output when Aw reaches a predeterminedvalue and maintains the output until the value of Aw approaches a smallvalue. The output of hysteresis circuit 122 is fed to the control logiccircuit 104 and particularly to a NOR gate 124 which has its outputconnected to a NOR gate 126. Assuming the latter gate 126 has no otherinput, the gate 126 will have an output triggering an amplifier 128which in turn will have an output on conductor 16 to open the releasevalve 28 whenever the hysteresis circuit 122 has a positive output.

The control logic circuit 104 further includes means for establishingthe deceleration threshold signal a A high level threshold input line:130 is energized by a positive pulse when brake pressure is firstmanually applied. Preferably, the input line 130 is connected to thebrake light circuit to be energized when the brake pedal is operated.The input line 130 leads to NOR gate 132 which has its output connectedto the input of NOR gate 134. The output of NOR gate 134 is applied byconductor 135 to the input of NOR gate 132 to hold the latter off andconsequently hold NOR gate 134 on for as long as the output of thehysteresis circuit 122 remains zero. The output of NOR gate 134 isconnected to a summer 1% which in turn is connected to the summer 114.As indicated diagrammatically, when the output of NOR gate 134 isenergized, a signal equivalent to a deceleration of -lg is applied tothe summer 136. In addition, a second input 138 to the summer 1136applies a constant signal equivalent to the deceleration of lg. Theoutput of the summer 136 will then be equivalent to 2g deceleration.

In operation, when the brakes are first applied, the output of thehysteresis circuit 122 will be zero momentarily and the high levelthreshold input 130 will be positive so that the NOR gate 132 will beturned off and the NOR gate 134 will be turned on. Then the output ofsummer 136 will be equivalent to -2g deceleration. This value of thedeceleration threshold is selected to insure that the wheel isdefinitely in an incipient lock-up condition before the electroniccontrol circuit becomes effective to modulate the brake pressure. Whenthe output of the hysteresis circuit 122 becomes positive, the NOR gate134 will be turned off to disable the first input of summer 136- tothereby switch the deceleration threshold 0: to a value equivalent to lgdeceleration. This value of a is maintained throughout the succeedingbrake modulating cycle. The ability of the circuit to change values ofthe deceleration threshold provides the system with a control functionheretofore unachieved.

The pressure hold circuit 106 The pressure hold circuit 106 isresponsive to wheel acceleration and after the wheel begins toaccelerate due to the release of brake pressure effected by the wheelvelocity circuit 102, provides a signal effective to hold the brakepressure constant by closing the hold valve 36. At the same time therelease valve 28 is closed. The pressure hold circuit 106 is quitesimilar to the wheel velocity error circuit 102. In particular, thewheel acceleration signal n: from the differentiator 112 is fed to asummer 140 where it is combined with an acceleration threshold signal 04The combined signal then passes through a summer 142 to an integrator144. The output Aw from integrator 144 is representative of the amountby which the wheel velocity exceeds a preprogrammed constantlyincreasing reference velocity which is a function of a A limit stopfeedback circuit 146 serves to inhibit positive output from theintegrator 144. Hence the IWVR principle is incorporated in the pressurehold circuit 106. A switching circuit 148 has Aw as its input and whenAw reaches a predetermined threshold value, provides a signal whichtriggers an amplifier 150 to produce an output on conductor 18 forclosing the pressure hold valve 36. The signal from the switchingcircuit 148 is also connected to the input of the NOR gate 126 to turnoff that gate thereby closing the release valve 28.

The control logic circiut 104 additionally includes means for adjustingthe acceleration threshold 0: There a NOR gate 152 has its inputconnected to the output of the hysteresis circuit 122 and has its outputconnected to summer 154 which has a second input 155 equivalent to anacceleration of 0.5g. The output of summer 154 then, supplies theacceleration threshold signal to the summer 140. As showndiagrammatically, the output of NOR gate 152, when it is turned on,provides an additional signal equivalent to 2g to the summer 154.Accordingly, then, the acceleration threshold a alternates between thevalue of 2.5g and 0.5g, depending whether the NOR gate 152 is turned on.The result of this arrangement is that when the output of hysteresiscircuit 122 is on, thereby indicating a substantial wheel velocity errorA01 the value of (1 will be small so that the pressure hold circuit 106will be quite sensitive to wheel acceleration when the wheel isrecovering from its incipient lock condition. This low value of a isselected to be as low as possible without making the pressure holdcircuit 106 sensitive to road noise which would give a false indicationof acceleration. The low value of a results in the brake pressure beingheld as high as possible during wheel acceleration to provide optimumbraking effect and therefore the optimum accelerating velocity profile.When, however, the wheel velocity has nearly recovered, Aw will havebecome quite small turning off the output of the hysteresis circuit 122and increasing the acceleration threshold a to a large value. The effectof this change in m is to rapidly drive Aw to a low value to effectopening of the hold valve 36 to thereby permit an increase of brakepressure.

The wheel velocity error circuit 102 The wheel velocity error circuit102, the control logic circuit 104 and the pressure hold circuit 106 areshown in schematic and logic form in FIG. 6. The summers 114 and 136 areshown in combination as input resistors 160, 162 and 164 connected to acommon point. The resistor 160 is connected to the input carrying thewheel acceleration signal to. The resistor 162 is connected to a voltagedivided 166 which is connected between a +3 v. source and ground. Thisvoltage divider 166 supplies the normal deceleration threshold signal (1The resistor 164 is connected to the output of the NOR gate 134 tosupply the increased deceleration threshold signal at the beginning ofthe brake modulation cycle. The integrator 118 comprises an operationalamplifier 168 of the type MCl530 for example, having a gain resistor 170and an integrating capacitor 172 serially connected between the outputand input thereof. A pair of diodes 174 in series are connected acrossresistor 170. A load resistor 176 is connected between ground and thejunction of resistor 170 and capacitor 172. The input of the operationalamplifier 168 is connected to the common point of the input resistors160, 162 and 164.

In operation, assuming that the deceleration threshold is set for lgwhich corresponds to 0.4 v., that voltage is supplied to the input ofthe operational amplifier 168 through the resistor 162. The operationalamplifier 168 output voltage is then at the negative saturation of 6.5v. When the amplifier is so saturated, any input value of a: which ismore positive than -0.4 v. will have no effect on the amplifier 168output (assuming that resistors 160 and 162 are equal). However, ifduring braking the vehicle wheel decelerates at a rate more than lg,then Li! will become more negative than 0.4 v. and the amplifier 168output voltage Aw will increase at a rate determined by the diflierenceof w and a and the integrator gain which is determined by resistors 160,170, 176 and capacitor 172 which have values of 100K, K, 2K and 0.1 mid.respectively. The component values are so selected that a drop in wheelspeed of approximately r.p.m. will cause the operational amplifier 168output Aw to increase from 6.5 v. to +0.8 v. Thus, the sensitivity ofthe integrator 118 to wheel change is very great giving accurate controlduring this range of operation. However, the memory capability of theintegrator 118 would be small if the same high gain were maintainedthroughout the integrator range. To increase the memory of theintegrator 118 it is designed to be non-linear. At the attainment of+0.8 v. level, the diodes 174 conduct and 7 short out the gain resistor170. This places one side of the capacitor 172 at the amplifier 168output and reduces integrator 118 gain by a factor of 11. The effect ofthis change in gain is to allow the integrator 118 to remember largedrops of wheel velocity such as occur during wheel lock-up conditions.For example, a drop in wheel velocity of 400 rpm. corresponds to theamplifier 168 output change from +0.8 v. to +6.5 v. which is thepositive saturation level of the operational amplifier 168.

The hysteresis circuit 122 of FIG. 4 is shown in FIG. 6 as a pair of NORgates 178 and 180, a transistor 182 and associated passive elements. Theoutput of the integrator 118 is fed through a resistor 184 to the NORgate 178 which is turn is connected to the NOR gate 126. The output ofthe NOR gate 178 is also connected to the input of the NOR gate 180. Theoutput of the latter is connected by feedback conductor 186 to an inputof the NOR gate 178. The transistor 182 has its base connected through alimiting resistor 188 to the output of the integrator 118. The emitterof transistor 182 is connected to biasing circuit comprising voltagedividing resistors 190 connected between a 7.5 v. supply and ground. Thecollector of transistor 182 is connected to a +7.5 v. supply throughvoltage dividing resistors 192. An input of the NOR gate 180 isconnected to the point between the voltage dividing resistors 192.

In operation of the hysteresis circuit 122, when the output Aw; of theoperational amplifier 168 increases from its negative saturation levelto 6.2 v., the transistor 182 becomes fully conductive causing an inputof the NOR gate 180 to become negative. At this point, however, theoutput of the NOR gate 178 is on so that the NOR gate 180 remains off.When the output of the operational amplifier 168 increases to +0.8 v.,the NOR gate 178 is turned off to open the release valve 28 via NOR gate126. The turning off of the NOR gate 178 causes the NOR gate 180 to turnon and hold the NOR gate 178 011. When the brake pressure has beenreleased enough to allow the wheel to accelerate the output voltage Awof the operational amplifier 168 will decrease. The NOR gate 178,however, will remain off until A011 falls below 6.2 v. causing thetransistor 182 to become non-conductive to consequently apply a positivevoltage to an input of the NOR gate 180 to turn it off. Then the NORgate 178 is enabled to turn on since both inputs have attained lowvoltage.

The circuit for establishing the deceleration threshold a comprises aconductor connected to a brake light switch 193 and to a capacitor 194which is connected to ground through a voltage dividing resistor 196.The capacitor 194 is also connected through a resistor 198 to the NORgate 132. When the brake light switch 193 is first closed, the capacitor194 transmits a positive voltage pulse or brake signal to the NOR gate132 to turn off that gate and to turn on the NOR gate 134, provided thatthe output of the NOR gate 180 is off. The deceleration threshold a isthen at 2g. The feedback on conductor to the NOR gate 132 holds thelatter off. When the NOR gate 178 turns 0ft" and the NOR gate 180 turnson, the NOR gate 134 will turn off to reduce the deceleration threshold(1 to lg.

The control logic circuit 104 further includes the NOR gate 152 havingits input connected to the output of the NOR gate 178. The output of theNOR gate 152 is connected through a bias network comprising, in series,voltage dividing resistors 200 and 202, a diode 204, and a voltagedividing resistor 206 connected at a point between resistor 200 anddiode 204 and to a 7.5 v. supply. When the NOR gate 152 is 011, anegative voltage equivalent to an acceleration of 2g is supplied throughdiode 204 and resistor 202 to the input of an operational amplifier 208.When, however, the NOR gate 152 is conducting, a positive output voltageis applied to the diode 204 to block conduction of the diode andaccordingly, to disable the 2g acceleration signal. A voltage dividingnetwork comprising resistors 210 and 212 is connected between a +3 v.supply and a 7.5 v. supply to provide through input resistor 214 voltagecorresponding to 0.5g acceleration. The input resistors 202 and 214 thencomprise the summer 154 of FIG. 4.

The pressure hold circuit The input circuit supplies the wheelacceleration signal to through input resistor 216 to the operationalamplifier 208 which is of the type MC1530 for example. Gain resistor 218in series with an integrating capacitor 220 are connected between theoutput and input of amplifier 208. A diode 222 is in parallel with theresistor 218 and a further gain resistor 224 is connected from the pointbetween the resistor 218 and the capacitor 220 and ground. The output ofamplifier 208 is connected through a limiting resistor 226 to a bufferamplifier 148 of type MC900 for example, which serves not only as aswitch responsive to a given voltage level but also as an amplifier andan inverter. The output of the buffer amplifier 148 supplies energizingsignals via the amplifier and the output conductor 18 to the hold valve36 and in addition, supplies an inhibit signal to the input of the NORgate 126 to close the release valve 28 when the hold valve 36 is closed.It will thus be seen that the operational amplifier and its associatedcircuitry is like the similar circuitry of the wheel velocity errorcircuit and comprises a non-linear integrator 144. A diode 228, however,is placed between the input of the operational amplifier 208 and groundto limit the negative input voltage to the integrator 144.

In operation, at the start of the brake pressure release period, NORgate 152 is turned on by reason of the NOR gate 178 being turned off.This action applies a positive voltage to diode 204 sufiicient to makethe diode nonconducting which reduces the acceleration threshold Aw from2.5g to 0.5g. When the wheel is decelerating,

causing a large negative voltage a, the operational amplifier 208 outputvoltage A012 is at the positive saturation level of +6.5 v. While thebrake pressure decreases,

but the wheel is still decelerating w remains negative holding theoutput Au of the amplifier 208 at +6.5 v. During this period, the diode228 limits the negative voltage at the amplifier 208 input to 0.1 v. toprevent the capacitor 220 from obtaining an excessive charge.

When the wheel acceleration signal at exceeds the +0.2 v. thresholdlevel a corresponding to 0.5g, the amplifier 208 output voltage startsdecreasing until the amplifier 208 output decreases to 0.4 v. Theintegrator 144 gain is determined by resistors 216, 218 and 224, andcapacitor 220 having values of 68K, 20K, 1K and 0.22 mfd. respectively.The components are selected such that a rapid increase in wheel speed ofapproximately 4 rpm. will cause the amplifier 208 output to decreasefrom +6.5 to +0.8 v. As the amplifier 208 output Aw drops below the +0.8v. level, the butter amplifier 148 output turns on to provide an outputsignal on the conductor 18 and close the hold valve 36. When Aw furtherdecreases to -O.4 v., the diode 222 conducts to short out the resistor218. This places one end of the capacitors 220 at the amplifier outputand reduces the integrator 144 gain by a factor of 21. The effect ofthis change in gain is to provide sufficient memory so that oscillatingwheel speed variations caused by vehicle suspension dynamic effects willnot cause the hold valve 36 to open during the brake pressure holdperiod. This memory capacity is equivalent to a transient wheel speedchange of about 80 rpm. When the wheel speed nearly recovers to approacha speed synchronous with vehicle speed, the NOR gate 178 is again turnedon which causes NOR gate 152 to turn oil causing diode 204 to conductand increase the acceleration threshold to 2.5g or +1 v. When the wheelacceleration signal in falls below the threshold level m the amplifier208 output Aw starts to increase. As the amplifier output voltage Awincreases above the 0.8 v. level, the buffer amplifier 148 turns offwhich opens the hold valve 36 allowing the brake pressure to increase.

System operation The operation of the brake system and the electronicsystem is graphically depicted in FIG. wherein the upper graph is a plotof brake pressure P vs. time, the middle graph is a plot of wheelacceleration to, the deceleration and acceleration thresholds a and aand the velocity error signals A40 and Aw vs. time, and the lower graphis a plot of wheel velocity to and vehicle velocity 11 vs. time. If thevehicle brakes are applied at time a the brake pressure P begins toincrease, and w as well as w and v begin to decrease. At time b thedeceleration Li drops below the value of the deceleration thresholdsignal a (the sum of the voltages applied through resistors 160, 162 and164 becomes negative) and the integrator begins to produce an output AwAt time c the value of Ana attains a value suflicient to trigger thehysteresis circuit 122 by turning off the NOR gate 178 and turning onthe NOR gate 126 to produce an output thereby detecting an incipientwheel lock-up condition. Then the amplifier 128 is turned on and therelease valve 10 28 is opened to relieve brake pressure, thedeceleration threshold signal a is reduced from --2g to -lg since theNOR gate 134 is turned on when the NOR gate 178 turns off, and theacceleration threshold signal a is reduced from 2.5g to 0.5g since theNOR gate 152 is turned on when the NOR gate 178 turns off.

With the brake pressure relieved I begins to increase toward zero. Thebrake pressure changes rapidly at first due to the action of theaccumulator 26. At time d the wheel acceleration 0; reaches Zero and thewheel velocity to begins to increase. After time dw exceeds theacceleration threshold (1 and the integrator 144 produces the signal AwAt time eAw becomes low enough to trigger the switch 148 to produce anoutput signal on conductor 18 to close the hold valve 36 to thereby holdthe brake pressure constant. At time 7 d1 decreases to a value less than:1 and Aw starts to increase. At time g the velocity error signal Awreaches the second switch point of the hysteresis circuit 122 where thetransistor 182 stops conducting thus turning off the NOR gate andturning on the output of NOR gate 178. The output of the NOR gate 178turns off the NOR gate .152 to change the acceleration threshold signalto a value of 2.5. g. This action causes the output Aw of the integrator144 to increase rapidly until at the time h Aw again operates the switch148 to remove the signal 18 and to open the hold valve 36 allowing thebrake pressure P to increase. By this time, the Wheel velocityapproaches synchronism with the vehicle velocity 11 but remains slightlyless than vehicle velocity since the most effective braking actionoccurs for that condition. After point h the cycle will repeat if wheeldeceleration a: again causes an incipient lock-up condition, but sincethe brake light switch remains closed, the NOR gate 132 remains off andthe deceleration threshold signal a will remain at the --l g value tomaintain the high incipient lock-up detection sensitivity for subsequentcycles.

In both the velocity error circuit 102 and the pressure hold circuit106, spurious input signals caused by dynamic suspension effects or byelectrical noise are suppressed by the integrators 118 and 144 whichtend to average and hence subdue the noise spikes. Moreover, since thebrake pressure release and hold signals on conductors 16 and 18 are notproduced until some time after the acceleration signal Ll! exceeds thethreshold signals (x and a an additional protection against suchspurious signals is provided. For example, between the times a and c,noise in the acceleration signal a; would not etfect switching of thehysteresis circuit 122 to produce a signal on conductor 16 at leastuntil very near time 0, even if some noise were carried over into theintegrated signal Aw It will be seen then that this innovation providesan antilock brake system having exceptionally rapid response to initiatecorrection of incipient wheel lock-up condition, and efifectingpseudo-proportional control of brake pressure with simple on-otf valves.The system further has the ability to vary the acceleration anddeceleration thresholds to optimize the control function.

The expression optimum wheel deceleration in the foregoing specificationand accompanying claims has reference to the 1g equivalent value of thethreshold in. It is optimum in the sense that is determined by thedesired rate of vehicle deceleration. Values other than lg may, ofcourse, be used.

The embodiments of the invention described herein are for purposes ofillustration and the scope of the invention is intended to be limitedonly by the following claims.

It is claimed:

1. In an antilock brake control system for a wheeled vehicle, means forsensing incipient wheel lock-up comprrsmg:

means for generating a first signal proportional to Wheel acceleration,

means for producing a variable deceleration threshold signal whichrepresents optimum wheel deceleration,

means for summing the first signal and the deceleration threshold signalfor producing a composite signal,

means for integrating the composite signal for producing an outputsignal, which output signal represents the amount that the time integralof the optimum wheel deceleration exceeds the actual wheel velocity,

means responsive to the output signal for producing a brake releasesignal,

the means for producing the deceleration threshold signal comprisingmeans for producing a brake signal when the vehicle brakes are firstapplied,

and means responsive to the brake signal and the brake release signalfor supplying a variable voltage to the summing means to change thevalue of the deceleration threshold signal when the brake release signalis first produced.

2. In an antilock brake control system for a wheeled vehicle, means forproviding a brake release signal comprising:

means for generating a first signal proportional to wheel acceleration,

means for providing a deceleration threshold signal which is a functionof the optimum wheel deceleration, means for summing the first signaland the deceleration threshold signal for producing a composite signal,

means for integrating the composite signal for producing an outputsignal which represents the amount that the optimum wheel velocityexceeds the actual Wheel velocity,

logic circuit means responsive to the output signal for producing abrake release signal when the output signal attains a predeterminedvalue,

the means for producing the deceleration threshold signal comprisingmeans for supplying a first voltage to the summing means,

means for producing a brake signal when the vehicle brakes are firstapplied,

second logic circuit means responsive to the brake signal for supplyinga second voltage to the summing means, and

means responsive to the brake release signal and connected to the secondlogic circuit means for inhibiting the second voltage thereby changingthe value of the deceleration threshold signal when the brake releasesignal is first produced.

3. In an antilock brake control system for a wheeled vehicle, means forsensing incipient wheel lock-up comprising:

means for generating a first signal proportional to wheel acceleration,

means for producing a variable deceleration threshold signal whichrepresents optimum wheel deceleration, means for summing the firstsignal and the deceleration threshold signal for producing a compositesignal,

means for integrating the composite signal for producing an outputsignal, which output signal represents the amount that the time integralof the optimum wheel deceleration exceeds the actual wheel velocy,

whereby the output signal is indicative of incipient wheel lock-up,

means responsive to the output signal for producing a brake releasesignal when the output signal attains a predetermined value,

means for generating a variable acceleration threshold signalrepresenting the optimum wheel acceleration after brake release,

a pressure hold means responsive to the first signal and theacceleration threshold signal for providing an output signal when wheelvelocity is increasing and exceeds a reference velocity by apredetermined amount, the reference velocity being a function of theacceleration threshold signal, wherein the pressure hold means includesan integrator having as an input signal a difference of the first signaland the acceleration threshold signal and having as an output signal theintegral of the said difference,

means responsive to the integrator output signal for producing apressure hold signal when the integrator output signal reaches apredetermined value,

and means responsive to the pressure hold signal for disabling the brakerelease signal when the brake hold signal is applied.

4. In an automatic brake control system for a wheeled vehicle anelectronic control circuit comprising:

means for generating a wheel signal proportional to wheel acceleration,

velocity error sensing means operative during vehicle braking andresponsive to the wheel signal for sensing incipient wheel slip and forproviding a brake release signal when wheel velocity becomes less than areference velocity by a predetermined amount,

means for generating an acceleration threshold signal representing theoptimum wheel acceleration after brake release,

a pressure hold circuit means responsive to the Wheel signal and theacceleration threshold signal for providing an output signal when wheelvelocity is increasing and exceeds a second reference velocity by apredetermined amount, the second reference velocity being a function ofthe acceleration threshold signal, wherein the pressure hold circuitmeans includes an integrating circuit, having as an input signal thedifierence of the wheel signal and the acceleration threshold signal andhaving as an output signal the integral of the said difference,

means responsive to the integrator output signal for producing apressure hold signal when the integrator output signal reaches apredetermined value, and

logic circuit means responsive to the pressure hold signal for disablingthe brake release signal when the pressure hold signal is applied.

5. An antilock brake control system for a wheeled vehicle havingmanually operated brake pressure means connected to brake meansincluding:

brake pressure modulating means for controlling brake pressure includingelectrically operated on-otf valve means,

transducer means for sensing wheel rotation and circuit means responsiveto the output of the transducer means for operating the on-off valvemeans comprising means connected to the output of the transducer meansfor developing an input signal,

means for producing a variable threshold signal which is a function ofthe optimum decelerating wheel velocity profile during braking,

means for comparing the input signal and the threshold signal forproducing an output signal representing the amount that the optimumdecelerating wheel velocity profile at any instant exceeds the actualwheel velocity,

and means responsive to the output signal for operating the on-oif valvemeans to reduce brake pressure when the output signal attains apredetermined value.

6. An antilock brake control system as defined in claim 5 wherein thecircuit means further comprises:

means for producing a second variable threshold signal which is afunction of the optimum accelerating 13 14 wheel velocity profile duringthe period of reduced References Cited brakmg UNITED STATES PATENTSmeans for comparing the input signal and the second threshold signal forproducing a second output representing the amount that the actual wheelvelocity 5 exceeds the optimum accelerating wheel velocity profile atany instant, DUANE, A. REGER, Primary Examiner and means responsive tothe second output signal for U S Cl XR operating the on-oflf valve meansto maintain the brake pressure when the second output signal attains 10a predetermined value.

3,245,727 4/1966 Anderson et al. 30321 3,498,682 3/1970 Mueller et a1.30321

